1. Field of the Invention
The invention relates to the field of cancer and, more specifically, to methods for detecting and treating microsatellite-instability positive (MSI(+)) tumors using the RIZ tumor suppressor gene.
2. Background Information
The retinoblastoma protein (Rb)-interacting zinc finger gene (RIZ) is a candidate tumor suppressor gene belonging to the PR (PRDI-BFl-RIZl homology) or SET (Suvar3-9, Enhancer-of-zeste, Trithorax) domain family of chromosomal regulators involved in chromatin-mediated gene activation and silencing. The PR/SET domain family plays an important role in human cancers as evidenced by genetic alterations of several members of this family. The PR domain of RIZ appears to be a protein-binding interface and can interact with a motif present in the C-terminal region of RIZ.
RIZ gene normally produces two protein products, RIZl and RIZ2, that differ at the N-terminal region by the presence or absence of the PR domain. The RIZl (PR+) product is considered a strong candidate for the tumor suppressor gene present on chromosomal region 1p36, a region commonly deleted in more than a dozen different types of human cancers. RlZl gene expression, but not RIZ2 expression, is commonly silenced in a variety of human tumors and tumor-derived cells, including those of breast, liver, colon, and neuroendocrine tissues (He et al., Cancer Res. 58:4238-4244 (1998); Jiang et al., Int. J. Cancer 83:541-547 (1999)). These tumors were characterized by inactivation of RIZ gene expression, rather than by mutation leading to altered RIZ protein structure. Forced RIZ1 gene expression in such tumor cells has been shown to cause G2/M cell cycle arrest, apoptosis, or both. However, the effect of RIZ1 expression in tumors in vivo has not been determined.
It is now commonly believed that cancers result from the accumulation of genetic alterations in cellular cancer-causing genes. These alterations are thought to be driven by genetic instabilities. Two major genetic instability pathways have been recognized in cancers, chromosomal instability (CIN) and microsatellite instability (MSI). The hallmarks of tumors of the CIN pathway are aneuploidy and loss of heterozygosity. In contrast, tumors of the MSI pathway are usually diploid and show massive instability in simple repeated sequences, or microsatellites.
Microsatellite instability is considered to result from defects in cells' DNA mismatch repair system. This system normally recognizes and restores misincorporated bases or slippage errors that occur during DNA replication. Loss of mismatch repair thus enhances the evolutionary process of mutagenesis and selection which underlies the development of cancer. The mechanism of tumorigenesis of MSI(+) tumors is thought to involve frameshift mutations of microsatellite repeats within coding regions of affected target genes whose inactivation directly contributes to tumor development.
In addition to mutation avoidance, DNA mismatch repair also plays a crucial role in determining the toxicity of a number of DNA-damaging agents that are used in cancer chemotherapy. For example, cell killing by methylating agents, such as N-methyl-N′-nitro-N-nitroguanidine, N-methyl-N-nitrosourea, streptozocin, temozolomide, and dacarbazine; by platinating agents, such as cisplatin and carboplatin; base analog drugs, such as 6-thioguanine; and other chemotherapeutic agents such as busulfan, etoposide and doxorubin, appears to require a functional mismatch repair system. Therefore, administration of chemotherapeutic drugs to patients with MSI(+) tumors may be ineffective.
Gene therapy with tumor suppressor genes is a simpler and less toxic alternative than chemotherapy or radiation. Several clinical trials are underway or proposed in which the tumor suppressor genes p53 or Rb are introduced into tumors which carry mutations in these genes, either using retroviral or adenoviral vectors (see, for example, Roth et al., Oncology 13S5:148-153 (1999). However, MSI(+) tumors generally do not carry mutations in p53 or Rb, and thus gene therapy with these genes is unlikely to be effective. To date, effective gene therapy methods for treating MSI(+) tumors have not been developed.
Accordingly, there exists a need to develop gene therapy approaches to treat MSI(+) tumors, and to develop reliable markers for determining the MSI status of tumors. The present invention satisfies this need and provides related advantages as well.